I recently updated to El Capitan after the constant notifications from Apple borderline forcing me to do so. It has performed quite well other than a few app incompatibility issues but they should be fixed by developers. Today however I ran into an issue when trying to watch a film, I went to the folder on my external HDD where I store the majority of my films etc. and none of the files were there, only a file called Thumbs.db. http://pasteboard.co/2Ex1UdII.png - (Link to image) I have no idea how this has happened apart from one theory, just after I updated OSX asked me if I wanted to use my external HDD as a time machine backup and I said yes at first. It then informed me that it would erase everything and asked me if I was sure, then I said no obviously. That's the only time the OS has ever mentioned my HDD and what's strange is every other file is still there. From what I remember I was in the middle of streaming from my Mac to my chromecast, watching a film on my TV but the file I was viewing is still there so that ruled out any potential corrupt file issues, also the file I was viewing was in a different folder anyway. Any help would be great.